The invention concerns security elements in the form of a strip which are in the form of a transfer film, in particular a hot stamping film and which can be applied to the surface of a substrate, in particular a value-bearing document such as a banknote, a cheque, a share certificate, a credit card and so forth or an identity card or pass, for safeguarding authenticity or quality of the substrate, comprising at least two plastic or lacquer layers between which is arranged at least one optically effective layer and/or structure producing an optical effect which is variable in dependence on the illumination or viewing angle.
Security elements of that kind are increasingly used in particular on banknotes but also on other documents which are to be suitably safeguarded in terms of quality or originality, but additionally also on articles. For example the low-value Euro banknotes (5, 10 and 20 Euro notes) carry corresponding, strip-shaped security elements in which an optically effective structure provides that the appearance of the corresponding strip-shaped security element changes in dependence on the illumination or viewing angle, for example it changes between the “C” symbol and other features. The strip-shaped security elements which are applied to the Euro banknotes are hot stamping films which include at least one generally thermally deformable protective lacquer layer into which the structure is embossed by means of a suitable master. The protective lacquer layer is usually transparent. If the optical effect of the structure is to be viewed in incident light, the structure is generally covered in the course of the stamping film manufacturing procedure by means of a layer which increases light reflection, for example a reflecting metal layer or a layer with a refractive index which is suitably different in relation to that of the protective lacquer layer. Then, there is applied to that combination of protective lacquer layer and reflection-enhancing layer at least one further lacquer layer, for example an adhesive layer, by means of which the hot stamping film can be fixed on the substrate. It is possible to forego the use of a separate adhesive layer if either a lacquer layer which also has adhesive properties is used or if the adhesive is applied to the substrate before applying the layer having the structure. Having regard to the various options referred to hereinbefore therefore it is to be understood that the term “plastic or lacquer layer” in accordance with the invention is used to denote not just layers of that nature in the strict sense but for example also films, adhesive layers and so forth.
An optical effect which is variable in dependence on the illumination or viewing angle can be produced in the most widely varying ways. Thus it is known for example to achieve motion effects by the use of suitable grating structures, as is described for example in EP 0 105 099 B1. Another possibility involves designing the optically effective structure in such a way that the viewer can recognise differing images in dependence on the illumination or viewing angle, as is the case with all Euro banknotes. The way in which that effect can be achieved in detail can be found for example in EP 0 375 833 B1. It is also known for a suitably variable optical effect to be achieved by means of holographic measures, in which respect recently holographic effects of that kind are generally also achieved by way of suitably configured spatial structures in respect of a separating layer between two plastic or lacquer layers.
A further possible way of achieving an optical appearance which changes in dependence on the illumination or viewing angle is discussed for example in WO 01/03845 A1. More specifically that document describes that in particular changes in colour can be achieved by the use of specific interference layer sequences or however also by using specific printing inks with interference pigments.
Besides the above-discussed strip-shaped security elements there are also a large number of areas of use where security elements in the form of delimited regions are preferred. That applies in particular if the aim is for the security element to be graphically incorporated into the surface configuration of the substrate. In addition design configurations in which the optically variable security element is provided only in given, closely defined regions, give a higher-value impression. In this connection reference may be made to the higher denominations of Euro banknotes (50, 100, 200 and 500 Euros) in each of which there is an optically variable security element in the form of a spatially delimited surface region.
Corresponding security elements are generally manufactured by way of coating plastic films as carrier webs. Those webs are then cut into strips which are then used for suitable transfer of the security elements onto the substrate.
If now the security element is in the form of a strip, that transfer operation does not cause any difficulties. It is generally possible to operate by means of pressure rollers of suitable width, in which respect the film webs having the security elements are cut into strips, prior to application to the substrate, with the width of the strips corresponding to the desired width of the security element strip.
It is also necessary to operate with films in strip form when applying security elements which are intended to cover only specific surface regions on the substrate. In that case however it is necessary to involve additional means which provide that, of a corresponding film web strip, only the respective corresponding surface region is applied to the substrate. When using security elements formed by hot stamping films there is on the one hand the possibility of using pressure tools, for example pressure rollers, which are provided with raised pressure elements which are respectively shaped to correspond to the surface region to be transferred. It will be appreciated that the production of such pressure rollers is considerably more expensive than the production of rollers with a smooth surface.
For that reason it has also already been proposed (see DE 35 11 146 A1) that stamping films without their own adhesive layer can be used and that adhesive can be provided on the substrate in the surface regions for receiving the security element, prior to transfer of the corresponding security element from the hot stamping film onto the substrate. That procedure affords the advantage that it is possible to operate with smooth stamping rollers or stamping wheels during the actual transfer procedure. However, the substrate has to be provided with adhesive in the desired region in a preceding printing operation, and that requires an additional working step and the use of suitable printing forms.
It was also already proposed (see DE 198 13 314 A1) that the layer sequence of a stamping film, which sequence is detachable from the carrier film and has the security element, can be subdivided into corresponding individual mutually separated surface regions, in which case then the adhesive layer is also provided only in those regions. The use of such a hot stamping film for applying the security elements to the substrate in predetermined surface regions also makes it possible to use smooth stamping rollers. The production of such hot stamping films however is comparatively complicated.
In addition, all the known methods of applying security elements to substrates in the form of delimited surface regions suffer from the common difficulty that the individual security element surface regions are usually arranged at a comparatively large spacing from each other on the substrate, for example a sheet with quite a number of banknotes. When transferring the security elements from a strip, that either means a comparatively large amount of waste or it requires the use of special measures which make it possible for one and the same starting strip with security elements to be passed over the substrate a plurality of times but in suitably displaced relationship, and thus, in a second or third pass over the substrate, to make use of the regions respectively remaining between transferred surface regions of suitably structured material or material which is adapted to be optically variable in some other fashion, to produce further security element surface regions. A corresponding apparatus is described in DE 42 42 105 A1.
DE 43 34 847 A1 discloses a value-bearing document with a window, in which the window is covered by means of a strip of a laminating film, which strip extends over the width of the document and is secured to the surface of the document. That laminating film strip includes a carrier film on which there are two lacquer layers, between which is arranged an optically effective structure in two definedly delimited surface regions. In that arrangement the one surface region is provided within the window while the other surface region is arranged over the substrate of the value-bearing document. The laminating film forming the strip is arranged in a recess in the surface of the substrate, which is desirably produced by the action of a suitably high pressure on the laminating film when applying the strip. In the case of the known value-bearing document it is absolutely necessary to use a laminating film because it is only that laminating film that is sufficiently stable on the one hand to reliably cover the window and on the other hand not to be damaged by the high forces involved when applying the strip. However there is no incentive for the man skilled in the art to use a corresponding strip if no window is to be covered over, especially as the provision of a recess generally alters the substrate in a mostly undesirable fashion.